In the field of automobile industry, FF-car production has recently been increasing for the purpose of reducing weight and expanding living space. 4WD car production is also growing due to their functionality. These FF and 4WD cars realize a power transmission and steering by the front wheels, and therefore, it is necessary to secure a smooth power transmission, even when the steering wheel is fully turned, and the constant velocity joint is inevitable as a part to transmit a rotational movement at a constant velocity in response to the changes in crossing angles between the crossing two axes.
Because performance of cars has recently been improved further and the production of high-power cars is increasing, constant velocity joints are exposed to greater stresses and severer lubrication condition.
Meanwhile, there is a tendency that an improvement of riding comfort in a car is also required at a higher level. The improvement of the riding comfort is demanded in any climate regions, for example, from extreme hot regions to extreme cold regions.
In extreme cold regions, the car may be started at an extreme low temperature. In such a condition, the rotational resistance may vary upon rotating due to the difference in frictional resistances among parts composing the constant velocity joint. Variation in rotational resistance may generate stick-slip sounds which may then lead to deteriorate the riding comfort.
There have been proposed grease compositions for constant velocity joints wherein a mineral oil is used as a base oil and a molybdenum compound is used as an additive (see e.g. Patent Documents 1 and 2). However, these grease compositions for constant velocity joints are incapable of sufficiently reducing variation in rotational resistance when different factors become combined at an extreme low temperature, therefore an improvement for more stable performance is required.    Patent document 1: JP-A-H10-273692    Patent document 2: JP-A-2003-165988